Phased array antennas have proven overly complex and expensive. The antennas use passive or active elements which produce a fixed amount of phase shift, and which must be independently switched into or out of the antenna feed structure and control network. This significantly increases cost, complexity, weight and size of the antenna system.
Current phased array imaging systems require each antenna element to have its own, low noise amplifier (LNA), narrow band filter, mixer and local oscillator (LO), in which the LO must be phase synchronized with all the antennas in the array. In a radio telescope array, the phase must be closed across of the aperture (length of the baseline). In order to produce an image, both the amplitude and phase distribution across the wavefront must be determined. Using a coherent source and receiving antennas placed on ½ wavefront intervals, discrete samples of the amplitude and phase can be collected and processed. By determining the magnitude and direction of the phase fronts, each phase center can be interpreted as a pixel in forming an image of sources in the far field (image quality would be dependent on the amount of coherence in the wave front).
The excessive requirements (size, weight, power and accuracy) for each antenna in the array make imaging phase array systems difficult to build. These systems often have a single receiving element at the focus of a parabolic reflector. Closing the phase across the array requires highly precise phase delay control electronics.
FIG. 1 is an example of a 25 element linear array showing 25 beam angles. Each beam angle is created by correlating the amplitude and phase information from all 25 antennas. For Nyquist sampling, each antenna must have a minimum phase sample resolution of (φ/50) radians, where each individual antenna has a beamwidth of φ radians. The amplitude and phase relationship across the array are correlated with angle-of-arrival (AOA) information to form an image of distant sources.
FIG. 2 depicts a four element linear phase array composed of discrete components. Each antenna requires a signal chain having a precise set of active and passive components. As shown, each chain includes a filter, an LNA, a phase shifter, and an LO, as mixer. The mixers require a complex and precise LO distribution network for temporal phase integrity. Each signal chain has its own phase shift network in order to align the phase of the incoming signal across the array.
On the other hand, as will be explained, the present invention provides a completely passive array with no requirements for a local oscillator or mixer, and no requirements for an intermediate frequency (IF) chain in each of the signal paths of the antennas. In addition, no preamplifiers are required and no phase shifters are required. In fact, the present invention provides for simpler and less expensive components than those components required for the phased array shown in FIG. 2.